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1. Millisecond exoplanet imaging: I. method and simulation results

   https://doi.org/10.1364/JOSAA.426046  

   Rodack, Alexander T. ; Frazin, Richard A. ; Males, Jared R. ; Guyon, Olivier ( September 2021 , Journal of the Optical Society of America A)

   One of the top priorities in observational astronomy is the direct imaging and characterization of extrasolar planets (exoplanets) and planetary systems. Direct images of rocky exoplanets are of particular interest in the search for life beyond the Earth, but they tend to be rather challenging targets since they are orders-of-magnitude dimmer than their host stars and are separated by small angular distances that are comparable to the classical$\mathrm{\lambda <\#comment/>}/D$diffraction limit, even for the coming generation of 30 m class telescopes. Current and planned efforts for ground-based direct imaging of exoplanets combine high-order adaptive optics (AO) with a stellar coronagraph observing at wavelengths ranging from the visible to the mid-IR. The primary barrier to achieving high contrast with current direct imaging methods is quasi-static speckles, caused largely by non-common path aberrations (NCPAs) in the coronagraph optical train. Recent work has demonstrated that millisecond imaging, which effectively “freezes” the atmosphere’s turbulent phase screens, should allow the wavefront sensor (WFS) telemetry to be used as a probe of the optical system to measure NCPAs. Starting with a realistic model of a telescope with an AO system and a stellar coronagraph, this paper provides simulations of several closely related regression models that take advantagemore »of millisecond telemetry from the WFS and coronagraph’s science camera. The simplest regression model, called the naïve estimator, does not treat the noise and other sources of information loss in the WFS. Despite its flaws, in one of the simulations presented herein, the naïve estimator provides a useful estimate of an NCPA of$\sim <\#comment/>0.5$radian RMS ($\approx <\#comment/>\mathrm{\lambda <\#comment/>}/13$), with an accuracy of$\sim <\#comment/>0.06$radian RMS in 1 min of simulated sky time on a magnitude 8 star. Thebias-corrected estimatorgeneralizes the regression model to account for the noise and information loss in the WFS. A simulation of the bias-corrected estimator with 4 min of sky time included an NCPA of$\sim <\#comment/>0.05$radian RMS ($\approx <\#comment/>\mathrm{\lambda <\#comment/>}/130$) and an extended exoplanet scene. The joint regression of the bias-corrected estimator simultaneously achieved an NCPA estimate with an accuracy of$\sim <\#comment/>5\times <\#comment/>{10}^{-<\#comment/>3}$radian RMS and an estimate of the exoplanet scene that was free of the self-subtraction artifacts typically associated with differential imaging. The$5\mathrm{\sigma <\#comment/>}$contrast achieved by imaging of the exoplanet scene was$\sim <\#comment/>1.7\times <\#comment/>{10}^{-<\#comment/>4}$at a distance of$3\mathrm{\lambda <\#comment/>}/D$from the star and$\sim <\#comment/>2.1\times <\#comment/>{10}^{-<\#comment/>5}$at$10\mathrm{\lambda <\#comment/>}/D$. These contrast values are comparable to the very best on-sky results obtained from multi-wavelength observations that employ both angular differential imaging (ADI) and spectral differential imaging (SDI). This comparable performance is despite the fact that our simulations are quasi-monochromatic, which makes SDI impossible, nor do they have diurnal field rotation, which makes ADI impossible. The error covariance matrix of the joint regression shows substantial correlations in the exoplanet and NCPA estimation errors, indicating that exoplanet intensity and NCPA need to be estimated self-consistently to achieve high contrast.

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2. Spectrally simplified approach for leveraging legacy geostationary oceanic observations

   https://doi.org/10.1364/AO.465491  

   Houskeeper, Henry F. ; Hooker, Stanford B. ; Cavanaugh, Kyle C. ( September 2022 , Applied Optics)

   The use of multispectral geostationary satellites to study aquatic ecosystems improves the temporal frequency of observations and mitigates cloud obstruction, but no operational capability presently exists for the coastal and inland waters of the United States. The Advanced Baseline Imager (ABI) on the current iteration of the Geostationary Operational Environmental Satellites, termed the$R$Series (GOES-R), however, provides sub-hourly imagery and the opportunity to overcome this deficit and to leverage a large repository of existing GOES-R aquatic observations. The fulfillment of this opportunity is assessed herein using a spectrally simplified, two-channel aquatic algorithm consistent with ABI wave bands to estimate the diffuse attenuation coefficient for photosynthetically available radiation,$K_d(\mathrm{P}\mathrm{A}\mathrm{R})$. First, anin situABI dataset was synthesized using a globally representative dataset of above- and in-water radiometric data products. Values of$K_d(\mathrm{P}\mathrm{A}\mathrm{R})$were estimated by fitting the ratio of the shortest and longest visible wave bands from thein situABI dataset to coincident,in situ$K_d(\mathrm{P}\mathrm{A}\mathrm{R})$data products. The algorithm was evaluated based on an iterative cross-validation analysis in which 80% of the dataset was randomly partitioned for fitting and the remaining 20%more »was used for validation. The iteration producing the median coefficient of determination ($R^2$) value (0.88) resulted in a root mean square difference of$0.319{\mathrm{m}}^{-<\#comment/>1}$, or 8.5% of the range in the validation dataset. Second, coincident mid-day images of central and southern California from ABI and from the Moderate Resolution Imaging Spectroradiometer (MODIS) were compared using Google Earth Engine (GEE). GEE default ABI reflectance values were adjusted based on a near infrared signal. Matchups between the ABI and MODIS imagery indicated similar spatial variability ($R^2=0.60$) between ABI adjusted blue-to-red reflectance ratio values and MODIS default diffuse attenuation coefficient for spectral downward irradiance at 490 nm,$K_d(490)$, values. This work demonstrates that if an operational capability to provide ABI aquatic data products was realized, the spectral configuration of ABI would potentially support a sub-hourly, visible aquatic data product that is applicable to water-mass tracing and physical oceanography research.

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3. Temperature dependence of collisional broadening and shift for the Kr 4 p ⁶ S 01→5 p [3/2] ₂ electronic transition

   https://doi.org/10.1364/AO.380102  

   Sahoo, Abinash ; Zelenak, Dominic ; Narayanaswamy, Venkateswaran ( February 2020 , Applied Optics)

   Temperature scaling of collisional broadening parameters for krypton (absorber)$4p^6{S}_0^1\to <\#comment/>5p[3/2{]}_2$electronic transition centered at 107.3 nm in the presence of major combustion species (perturber) is investigated. The absorption spectrum in the vicinity of the transition is obtained from the fluorescence due to the two-photon excitation scan of krypton. Krypton was added in small amounts to major combustion species such as${\mathrm{C}\mathrm{H}}_4$,${\mathrm{C}\mathrm{O}}_2$,${\mathrm{N}}_2$, and air, which then heated to elevated temperatures when flowed through a set of heated coils. In a separate experimental campaign, laminar premixed flat flame product mixtures of methane combustion were employed to extend the investigations to higher temperature ranges relevant to combustion. Collisional full width half maximum (FWHM) ($w_C$) and shift (${\mathrm{\delta <\#comment/>}}_C$) were computed from the absorption spectrum by synthetically fitting Voigt profiles to the excitation scans, and their corresponding temperature scaling was determined by fitting power-law temperature dependencies to the$w_C$and${\mathrm{\delta <\#comment/>}}_C$data for each perturber species. The temperature exponents of$w_C$and${\mathrm{\delta <\#comment/>}}_C$for all considered combustion species (perturbers) were$-<\#comment/>0.73$and$-<\#comment/>0.6$, respectively. Whereas the temperature exponents of$w_C$are closer tomore »the value ($-<\#comment/>0.7$) predicted by the dispersive interaction collision theory, the corresponding exponents of${\mathrm{\delta <\#comment/>}}_C$are in between the dispersive interaction theory and the kinetic theory of hard-sphere collisions. Comparison with existing literature on broadening parameters of NO, OH, and CO laser-induced fluorescence spectra reveal interesting contributions from non-dispersive interactions on the temperature exponent.

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4. Pressure scaling of the collisional broadening parameters of Kr 4 p ⁶ S 01→→5 p [3/2] ₂ transition

   https://doi.org/10.1364/AO.394932  

   Sahoo, Abinash ; Zelenak, Dominic ; Narayanaswamy, Venkateswaran ( September 2020 , Applied Optics)

   Pressure scaling of collisional broadening parameters of krypton (absorber)$4p^6{S}_0^1\to <\#comment/>\to <\#comment/>5p[3/2{]}_2$transition centered at 107.3 nm in the presence of nitrogen${\mathrm{N}}_2$(perturber) is investigated. The absorption spectrum in the vicinity of the transition is obtained from the two-photon excitation scan of krypton in the presence of the perturber at different prescribed pressures varying from a few torrs to 10 atm. The absorption spectra reveal noticeable asymmetry at atmospheric pressure, and the asymmetry becomes increasingly pronounced with pressure; however, the absorption spectra at sub-atmospheric pressures tested are symmetric. The absorption spectra are fitted with synthetic asymmetric Voigt profiles across all pressures, wherein the asymmetry parameter is varied to capture the asymmetry at different pressures. The collisional shift (${\mathrm{\delta <\#comment/>}}_C$), the symmetric equivalent collisional full width at half maximum ($w_{C,0}$), and the asymmetry parameter ($a$) are determined from the synthetic fits at various pressures. All the parameters are observed to vary linearly with pressure over the entire range of the pressure values tested. The mechanisms that cause the asymmetry in the absorption spectra are also discussed.
5. Spectroscopic study of the 4 f ⁷ 6 s ^{2 8} S 7/2∘−4 f ⁷ ( ⁸ S ^∘ )6 s 6 p ( ¹ P ^∘ ) ⁸ P _9/2 transition in neutral europium-151 and europium-153: absolute frequency and hyperfine structure

   https://doi.org/10.1364/JOSAB.467968  

   Herd, M. T. ; Maruko, C. ; Herzog, M. M. ; Brand, A. ; Cannon, G. ; Duah, B. ; Hollin, N. ; Karani, T. ; Wallace, A. ; Whitmore, M. ; et al ( September 2022 , Journal of the Optical Society of America B)

   We report on spectroscopic measurements on the$4f^{7}6s^2{}^8{S}_{7/2}^{\circ <\#comment/>}\to <\#comment/>4f^7{(}^8{S}^{\circ <\#comment/>})6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$transition in neutral europium-151 and europium-153 at 459.4 nm. The center of gravity frequencies for the 151 and 153 isotopes, reported for the first time in this paper, to our knowledge, were found to be 652,389,757.16(34) MHz and 652,386,593.2(5) MHz, respectively. The hyperfine coefficients for the$6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$state were found to be$\mathrm{A}(151)=-<\#comment/>228.84(2)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(151)=226.9(5)\mathrm{M}\mathrm{H}\mathrm{z}$and$\mathrm{A}(153)=-<\#comment/>101.87(6)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(153)=575.4(1.5)\mathrm{M}\mathrm{H}\mathrm{z}$, which all agree with previously published results except for A(153), which shows a small discrepancy. The isotope shift is found to be 3163.8(6) MHz, which also has a discrepancy with previously published results.